This invention relates to lasers having a plurality of optical cavity modes which oscillate under the gain curve, and to stabilizing the frequencies of those modes so that those frequencies do not vary appreciably with time. More particularly, the invention is concerned with the mode stabilization of lasers, particularly gas lasers, having three or more longitudinal modes, which in the absence of other qualifying language should be taken to mean that the modes share the same transverse mode indices m and n, and differ only in their longitudinal mode index q (see Equation 1 below). Multiple transverse configurations may also be treated as a simple logical extension of the longitudinal case, to which this discussion is mainly particularized for the sake of clarity and concreteness.
A number of types of lasers, particularly gas lasers, exhibit several longitudinal modes. The stabilization of a laser having three or more modes can be difficult, since these may be relatively close in frequency, and until the present invention, there was not a convenient property of the laser's output radiation which could be monitored and used in a simple way to feed back a control signal to hold the modes stable in frequency.
Lasers often exhibit a problem in which the longitudinal mode frequencies vary with time. This is caused by variations in optical cavity length, which are often thermal or acoustic in origin. Without any stabilization mechanism, such a laser will have modes whose positions drift back and forth under the gain curve, so that the output frequencies emerging from the laser will vary. This can cause problems in several respects. First, the laser may be desired for use with a particular stabilized frequency performance characteristic, as a requirement of a certain type of testing or end use. Mode stability can be important in laser Doppler velocimetry applications. Also, in cases where a laser beam is to be modulated with signal information and then demodulated at a later time, the constancy of the individual mode frequencies can be a great advantage. Secondly, if mode frequencies are allowed to vary in relation to the frequency of the natural atomic transition frequency responsible for the emission (and hence vary in relation to the gain curve of the laser medium), the total intensity of the laser will fluctuate as a result. In many uses of lasers, it is important to have a stable total output intensity. Thus, this technique also provides a measure of intensity stabilization.
The optical frequencies f.sub.mnq associated with the transverse electromagnetic modes TEM.sub.mnq which a laser resonator of optical cavity length L can support are given by EQU f.sub.mnq =F{q+(1+m+n) .pi..sup.-1 arccos [(1-L/b.sub.1) (1-L/b.sub.2)].sup.1/2 } [Eq. 1]
where:
b.sub.1 and b.sub.2 are effective mirror radii of curvature PA0 q is the longitudinal mode number PA0 m and n are transverse mode numbers PA0 F is the "free spectral range" given by C/(2 L) PA0 C is the speed of light PA0 L is the optical cavity length where ##EQU1## L.sub.i are physical length elements along a chief ray connecting the cavity mirrors, and PA0 n.sub.i are the local refractive indices for each L.sub.i.
The present invention is concerned with stabilization of laser frequencies to accuracies better than one half of a free spectral range as defined above.
There have been longitudinal mode stabilization systems for lasers having two modes. Two mode lasers have resonator tubes or cavities which are shorter in length than the three or more mode lasers with which the present invention is concerned. For example, in a helium-neon laser of 23.4 cm length there will be two modes under the gain curve separated by 640 megahertz (one free spectral range). In a tube with no inherent polarization mechanism (such as a Brewster window), each mode often assumes a pure linear polarization state which is orthogonal to that of the other mode, and these polarization orientations remain stable in time. This particular behavior of the modes with respect to polarization is a function of the birefringence of optics which are in the laser cavity, e.g. the mirrors.
A common method which has been used for stabilization of the mode frequencies in such a two mode laser (one with orthogonal polarizations) has been to balance the intensities of the two modes. An external polarizing beamsplitter can be used to separate the two modes based on their polarization properties. With the two modes separated according to polarization state, each mode is directed onto a detector, and the resultant signals from the two detectors are compared to assure that relative light intensities of the two modes remain constant. Any deviation from the set point can be made to generate an error signal. The error signal can be used to control a heater surrounding the tube to adjust the tube's length by changing its temperature (and hence its length via its thermal expansion). This results in the return of the modes to their original displacements from line center, thus enforcing frequency stability.
Stabilization of lasers with three or more longitudinal modes is much more troublesome. If the laser is inherently polarized (due perhaps to intracavity optics such as a Brewster plate), then the outputs at the different optical frequencies f.sub.mnq for several adjacent q's are no longer separable based on their simultaneously distinct and distinguishable polarization properties, as they are all the same polarization. Alternatively (as in the case for two mode lasers), multiple longitudinal mode lasers without intracavity polarizers most often will lase such that the output for a given longitudinal mode index q is linearly polarized. Furthermore the various modes for which oscillations occur, denoted by q's, will have one of two orthogonal polarizations, also as in the two mode case. It is not predictable a priori, however, which modes will have a given polarization or for that matter how many modes will exist in each of the two possible polarization states. (Usually, for a specific tube, it is found that modes for adjacent q's have orthogonal polarizations. This behavior, however, is far from universal, and is extremely prone to disruption by minor perturbations in operating conditions, including plasma tube temperature.)
Prior to the present invention it was not apparent how to stabilize a multimode laser in a simple and convenient way. It is among the objects of the present invention to address the monitoring of the frequencies of optical modes in a laser by utilizing a known property or effect which occurs when the light from a laser with three or more modes is monitored with a photodetector, and to use this information to control the laser, e.g. by adjusting the optical cavity length based on a control signal derived from monitoring this property.